Despite his firm belief in the timeless existence of a spiritual and civilizational entity called "India," Jawaharlal Nehru nevertheless felt compelled to begin his appropriately titled Discovery of India with a solid and physicalistic description of her "natural" frontiers. Nehru's imaginative geography depicted impassable mountain ranges, vast deserts, and deep oceans that produced a "natural" cradle for what became India. Anxiety regarding the physical boundaries of the nation gets inscribed early in Nehru's Autobiography. The narrative script that runs through that definitive work in imagining India clearly traces her downfall to porous frontiers and, more importantly, to an unfortunate timing by which disunited and fragmenting India encountered the cresting and united civilization of the British. The encounter not only produced colonial rule but also with it, Nehru argued, the sources of India's eventual redemption: modernity, science, the rational spirit, and, most importantly, national unity.Notice the tension between, on the one hand, the reference to "impassable" mountains and "vast" deserts and, on the other hand, the reference to "porous frontiers." The mountain ranges clearly weren't impassable enough, nor the deserts vast enough, to prevent multiple conquests of the subcontinent.
The idea of 'natural frontiers' has a long and somewhat checkered history. Although natural features of the landscape do play some role in how the territorial boundaries of states have evolved, that role I think is a secondary or even tertiary one -- that is, I incline to the view that it's secondary in terms of boundaries' actual on-the-ground history, as distinguished from the often larger role 'natural frontiers' play in the legitimating myths of some nation-states.[**]
*Sankaran Krishna, "Cartographic Anxiety: Mapping the Body Politic in India," orig. published in Alternatives v.19 n.4 (Fall 1994), reprinted in Challenging Boundaries, ed. M. Shapiro and H. Alker (U. of Minnesota Press, 1996), pp. 193-214. The quotation is from p.195 (endnotes omitted).
**The relevant literature is fairly extensive and I won't go into it here (though I'm probably willing to do so in the comments if someone wants).